Windows Vista and 7 come with quite good content blockers and log web usage too, but it starts becoming a pain for homework when a lot of un-rated sites are blocked. You can go for subscribed software that uses whitelist/blacklist methods, but there is a continual expense. Definitely don't run an administrator account as the main issue is not naughty content but nasty viruses from drive-thru websites, that are geared towards young inquisitive minds.